調用JNI方法時如何獲取Android上下文實例？

Question

我已經在stackoverflow上閱讀了一些相關的答案，但似乎沒有人回答我的問題。我將從本機代碼中獲取android ID，即在C代碼中調用getAndroidIDfromNativeCode方法，（因此JVM通過方法在本機代碼中初始化create_vm)，你知道調用getContentResolver方法時，必須使用Android Context實例來調用，那么如何獲取這個Context實例呢？

    static jstring
    native_code_getAndroidID(JNIEnv *env, jobject thiz)
    {
        jclass c_settings_secure = (*env)->FindClass(env, "android/provider/Settings$Secure");
        jclass c_context = (*env)->FindClass(env,"android/content/Context");
        if(c_settings_secure == NULL || c_context == NULL){
            return NULL;
        }
        //Get the getContentResolver method
        jmethodID m_get_content_resolver = (*env)->GetMethodID(env, c_context, "getContentResolver",
                                                               "()Landroid/content/ContentResolver;");
        if(m_get_content_resolver == NULL){
            return NULL;
        }
        //Get the Settings.Secure.ANDROID_ID constant
        jfieldID f_android_id = (*env)->GetStaticFieldID(env, c_settings_secure, "ANDROID_ID", "Ljava/lang/String;");

        if(f_android_id == NULL){
            return NULL;
        }
        jstring s_android_id = (*env)->GetStaticObjectField(env, c_settings_secure, f_android_id);

        //create a ContentResolver instance context.getContentResolver()
        /*
          where can I get the context instance from Anroid APP??
        */
        jobject o_content_resolver = (*env)->CallObjectMethod(env, context, m_get_content_resolver);
        if(o_content_resolver == NULL || s_android_id == NULL){
            return NULL;
        }
        //get the method getString
        jmethodID m_get_string = (*env)->GetStaticMethodID(env, c_settings_secure, "getString",
                                                           "(Landroid/content/ContentResolver;Ljava/lang/String;)Ljava/lang/String;");

        if(m_get_string == NULL){
            return NULL;
        }
        //get the Android ID
        jstring android_id = (*env)->CallStaticObjectMethod(env, c_settings_secure,
                                                            m_get_string,
                                                            o_content_resolver,
                                                            s_android_id);
        return android_id;
    }

    JNIEnv* create_vm() {
      JavaVM* jvm;
      JNIEnv* env;
      JavaVMInitArgs args;
      JavaVMOption options[1];

      /* There is a new JNI_VERSION_1_4, but it doesn't add anything for the purposes of our example. */
      args.version = JNI_VERSION_1_4;
      args.nOptions = 1;
      options[0].optionString = "-Djava.class.path=-jar-path";
      args.options = options;
      args.ignoreUnrecognized = JNI_FALSE;

      JNI_CreateJavaVM(&jvm, (void **)&env, &args);
      return env;
    }

    char *jstringTostr(jstring android)
    {
      ...
    }

    //I will call this function from native code.

    char *getAndroidIDfromNativeCode()
    {

      JNIEnv* env = NULL;

      env = create_vm();
      jstring androidID = native_code_getAndroidID(env,NULL);
      return jstringTostr(androidID);

    }

Answer 1

下面的方法可以獲得一個Context 實例。

static jobject getGlobalContext(JNIEnv *env)
{

    jclass activityThread = (*env)->FindClass(env,"android/app/ActivityThread");
    jmethodID currentActivityThread = (*env)->GetStaticMethodID(env,activityThread, "currentActivityThread", "()Landroid/app/ActivityThread;");
    jobject at = (*env)->CallStaticObjectMethod(env,activityThread, currentActivityThread);

    jmethodID getApplication = (*env)->GetMethodID(env,activityThread, "getApplication", "()Landroid/app/Application;");
    jobject context = (*env)->CallObjectMethod(env,at, getApplication);
    return context;
}

Answer 2

jobject類型的最后一個參數是您在其中保留 Native 方法聲明的類的實例。

因此，如果您在活動中聲明您的方法，您可以直接使用最后一個參數thiz作為您的上下文實例。

否則，您將需要在本機聲明中添加另一個Context類型的參數，並在方法定義中添加jobject類型的另一個參數。